Methane and Global Warming.
Until recently the general public associated the problem of global warming mainly with the build up of carbon dioxide in the atmosphere. Climate researchers have known for years that other gases are also involved, and that some of these have more heat-trapping capability than carbon dioxide. Methane is a prominent example. A molecule of methane has twenty-one times the heat-trapping capacity of a molecule of carbon dioxide (EPA, 2010). In recent years the general public has become more aware of the impact of methane, based on the publication of articles in a number of popular publications, including The New York Times (Kaufmann, 2009) and the websites of Scientific American (Mims, 2010) and Time (Walsh, 2010)
Methanogens.
Methane usually originates from the activity of specialized microorganisms, called methanogens, which degrade organic matter in the absence of oxygen. Cattle rumens, landfills and swamplands are the primary habitats of methanogens. Environmental Protection Agency (EPA, 2010) figures indicate that only enteric fermentation (i.e., digestive processes of ruminant animals) produces more methane emissions in the United States than landfills.
Inhibition of Methanogenesis.
Methanogens are not the only specialized microbes in the environment which can degrade organic matter in the absence of oxygen. They have to compete with several other types of microbes to gain access to the compounds which they need to generate energy, and methane. These other microbes can generate energy more efficiently than methanogens can, and they do so without producing methane. When the key nutrients required by these other groups are present, they will out-compete the methanogens, and methane output will be low. Thus, environments containing nitrate, sulfate, manganese(IV) or ferric ions generally do not support extensive formation of methane (Konhauser, 2007).
An example of the importance of competing modes of metabolism can be seen in the effect of the sulfate ion. Sulfate is an important inhibitor of methanogenesis. A metabolic class of microbes known as the sulfate-reducing bacteria (SRB) are abundant in the environment, and wherever sulfate is present in an anaerobic environment they will efficiently metabolize it and out-compete the methanogens. Quantities of sulfate are low in most freshwater sediments. As a result, SRBs are largely inactive, and methanogenesis can take place unimpeded.
On the other hand, marine sediments contain high levels of sulfate, SRBs are very active, and methanogenesis is strongly inhibited. The presence of sulfates in wetlands has major implications for the global climate. Gauci et al. (2004) concluded that man-made sulfur pollution has reduced the formation of methane in wetlands by about 8% worldwide, compared to the total that would have been formed in the absence of the sulfur pollution.
In addition to the SRBs, methanogens must also compete with microorganisms capable of producing energy by reducing nitrate, manganese or iron (Konhauser, 2007). When any of these other microorganisms have access to their essential nutrients, they are likely to out-compete the methanogens, and methane formation will be severely limited.